Current methods used to fabricate superconducting coils are described in European patent application 02 755 238.9, first published as a PCT patent, application number PCT/GB02/03898, on 6 Mar. 2003 and entitled “Superconducting Coil Fabrication”. That application disclosed a method of fabricating a superconducting coil. That method comprises a first step of fabricating individual coil tracks by depositing, shaping and texturing superconductive material using film deposition and patterning techniques, in situ, in individual deposited layers on a former which has a substantially curved surface. The method comprises further steps of testing, in situ, each coil track in terms of texture or superconducting performance and fabricating the coil tracks whereby each deposited layer is patterned by a masking, or marking, operation before, or after, layer deposition. The testing step, therein described, is used at specified steps of the fabrication process. However, it is used only to test whether the coil superconducts, or has appropriate texture for having superconductive properties. Each coil, therefore, passes or fails that testing step. Even if the probability of fabricating a coil track which fails this test is low, the chances of having one or more failed coil tracks in the superconducting coil increases with an increase in the number of layers to the coil, so reducing effectiveness of the fabricated coil and increasing the wastage produced in the fabrication process.
A superconducting coil track may fail the testing step if the coil track comprises a serious defect. That defect can be of a number of types: a repairable defect; an irreparable defect; and a defect, of either repairable and irreparable form, that propagates through successive layers by virtue of a copying process. Therefore, a coil fabricated by the current method is unusable if one layer has a propagating defect, as all further layers would have that same defect and would fail the testing step.
The field created by the fabricated superconducting coil is also defined by the configuration of the coil, and therefore the coil tracks that comprise the coil, and the current passing through the coil. For a current to pass through the coil, all parts of each coil track must allow passage of that current; that is to say the components of the track which comprise each coil track are in series. The final current which can be passed through the coil is limited by the weakest link in the coil. The weak links in the coil are caused by, amongst other causes (such as poor connections between coil tracks), defects in the layers.
Further, the geometry of the field generated by a coil track having a specific geometry and a defect will differ from the field generated by a coil track having exactly the same properties, but without the defect. The field differs not only because a different current passes through the coil track, but because the physical geometry of the comparable parts of the two coil tracks differ in the locality of the defect, and the defect may comprise a chemical impurity or the material in the locality of the defect may have a crystalline or lattice structure different from the rest of the material in the coil track. The defect may cause the coil track to exhibit different physical characteristics from the coil track without defects, in superconducting conditions. Therefore, as successive coil tracks are created on the former, with each coil track having its own unique defects, the shape of the field created by the coil varies from that of a coil created from coil tracks that do not have any defects.
If the path of a coil track is varied to avoid irreparable defects in its layer, the shape of the field produced by the coil in superconducting conditions is also varied. However, as a given coil track can be defined onto its layer so as to avoid defects, the path of that coil track can be adapted to rectify the shape of the field produced under superconducting conditions by that coil track, together with the fields from other coil tracks underneath that coil track.
Although the known method refers to a testing step for each coil track before fabrication of the coil is continued, the testing is used only to determine whether the coil track works, and not to locate defects in the coil track for repair or avoiding. The defects in the layer are not identified or repaired. Further, the path of the coil track defined onto, or into, the layer is not varied to avoid the weak links, irreparable defects, and to rectify the produced superconducting field, but to provide a specific geometry of coil configuration.
The known fabrication process can be improved to increase the proportion of working coil tracks fabricated by means of the fabrication process: by identifying whether each defect present in a layer is repairable; repairing the repairable defects; choosing a path of coil track that avoids the irreparable defects; calculating the effect on the geometry of the produced superconducting field by that coil track and to rectify the shape of the superconducting field corresponding to underlying coil tracks; amending the chosen path of coil track to account for these effects to the produced superconducting field; and defining the path into or onto the layer in order to create the coil track. Further, before the fabrication procedure continues with the deposition of a further layer, so long as the layer is a superconducting layer, the coil track can be tested to ensure it superconducts.
The aim of the present invention is to provide such an improved method for fabricating superconducting coils by means of a testing step.